The present invention relates to IC cards and more particularly to technology for contactless IC cards which transmit and receive signals and supply electric power by electromagnetic induction.
In recent years, the use of contactless IC cards has been rapidly spreading in a wide range of fields including electronic transportation tickets, credit cards and ID cards because they can be conveniently used to read and write data without the need for insertion into an exclusive machine for IC cards.
A contactless IC card has an antenna coil and an IC chip embedded in a card body and employs electromagnetic induction technology to transmit and receive signals and supply electric power using electromagnetic waves from an external reader/writer.
The circuitry of the IC chip embedded in the card body includes a modem for modulating and demodulating a signal from a reader/writer, a control circuit for performing processing according to the signal, a power supply circuit for obtaining operating power, and a memory circuit for storing data.
JP-A No. 2007-281818 discloses a contactless IC card which securely and efficiently obtains required electric power depending on the distance from a reader/writer and exchanges required data with the reader/writer.
In this contactless IC card, a small antenna coil is located almost in the center of the card body and a large antenna coil, which has an antenna length larger than the small antenna coil, is located outside it. The two antenna coils are placed on a plane and an IC chip, electrically coupled to the two antenna coils, is located between them. The two antenna coils are disposed so that their winding directions are opposite to each other, which means that the phases of currents flowing in the two antenna coils are opposite and interference does not occur between the coils.
This contactless IC card makes a comparison as to whether the composite voltage of first received voltage from the small antenna coil and second received voltage from the large antenna coil is larger or smaller than a reference supply voltage. When the distance between the card and reader/writer becomes shorter than a prescribed distance, the large antenna coil is turned off and only the small antenna coil is activated, thereby preventing breakdown of the IC chip. On the other hand, when the distance from the reader/writer becomes longer than the prescribed distance, the large antenna coil is again turned on to obtain a high level of composite voltage so that power supply and data reception from the reader/writer are stably performed.
In the contactless IC card described in JP-A No. 2004-348636, a pair of electromagnetic induction coils which function as magnetic antennas are provided on a synthetic resin card base. These electromagnetic induction coils are placed on a plane and they are opposite in winding direction and equal in the number of turns.
In this case, when the distance between the contactless IC card and the reader/writer becomes shorter than the prescribed distance, an electromotive force twice larger than that generated by a single electromagnetic induction coil is obtained by interaction of the electromagnetic induction coils. Therefore, larger electric power is supplied to the IC chip of the contactless IC card, so the CPU can process complicated and high levels of calculations. Even if a disturbance magnetic flux is applied to the reader/writer in one direction, the disturbance magnetic flux can be cancelled so that communications between the contactless IC card and the reader/writer are stable.
The contactless IC card reader/writer described in JP-A No. 2001-313515 has an antenna section which includes two circular loop antennas with different radii. The two loop antennas are located so that their centers coincide, and ends of them are coupled so that currents flow through them in opposite directions. The distance of each of the two loop antennas from the center is fixed so that the intensity of a composite magnetic field which combines magnetic fields generated by the loop antennas does not increase at a short distance. This ensures that the intensity of the magnetic field generated by the antenna section is independent of the communication distance and maintained at a constant level so that it can stably communicate with an IC card at a short distance and can also communicate with an IC card at a remote distance.